VHF help needed

[john_travers]

Why can I hear the coast guard (Liverpool) loud and clear when they transmit on 16 but not on any other channel?
Channel 86 seems to be a particular offender. Lots of background noise. Almost unreadable.
I have just fitted a new masthead whip Ariel from Simpson Lawrence. The type that relies on stripping the co-axial back, pushing into the base of the ariel and hoping you get a good connection.
I can transmit OK but can only be heard at relatively short distances. Are the two problems connected ? How do you check? What should I check?

 

[ChrisSnushall]

You should be able to receive and transmit at the same distance; more or less line of sight. If you can receive from Liverpool Coastguard there is nothing wrong with your aerial.

To check - call up Liverpool Costguard on Channel 16 and ask for a radio check. They may tell you to go to Channel 67. If so, go to Channel 67 and call them again. Obviously if you can hear them and they can hear you there is no problem.
They will tell you if your signal is weak or 'hearing you loud and clear'.
Often when listening on Channel 16 you will hear the Coastguard talking to other yachts but you won't be able to hear the other yacht reply; this is because you are out of range of their transmitter - in other words their aerial is over the horizon from your aerial; but you are still in range of Liverpool Coastguard. You may be 25 miles to the north of the Coastguard Station - within range - and the other vessel 25 miles south of the coastguard station. Both within range to be heard by the Coastguard but not within range to talk to each other.
Hope this helps a little.

 

[tom_bishop]

When the Coastguard service began transmitting weather forecasts on Channels 10 and 73 instead of Channel 67, I noticed that reception was not as good, and in particular the forecasts could no longer be heard at the same distance as previously. I took the opportunity to query this with a member of the Coastguard service at the Southampton Boat Show that year and he advised that they rent aerial space on masts owned by, for example BT. On these masts, their aerials are at various heights, with their highest aerial being allocated to Ch. 16, the next to Ch. 67 and aerials allocated to other channels being lower down the mast. This may explain your first problem, as the higher the aerial the greater the distance at which you can achieve line of sight (although you don't say how far you are from the CG aerials or whether there are any obstructions to the signal).
I cannot see that a poor coax connection would affect reception more on one channel than on another.

 

[john_travers]

Thanks for the advice, I did'nt know coast gaurd ariels were at different hieghts dependand on channel used. However, I think the problem is not a line of sight one as other boats anchored in the same vacinity can pick up Liverpool on 86 no problem. I can receive using an emergancy ariel connected to the main VHF. I can even receive better on the hadheld!
All this would suggest an ariel problem, I think. But if that is true should I be able to transmitt?

 

[Strathglass]

To check correctly yhou need a VSWR meter this will sheck that myour aerial matches correctly across all the bands.

 

[Boatman]

Before I comment on how to solve the problem you should be aware of some of the basic concepts that are involved with any radio transmission. For any given channel there is a fixed wave length which dictates the exact length of a matched antenna for good reception and transmission. For example if you match an antenna for Ch 16 it will function less well the further the frequency move away from the Ch 16.

Most antenna problems show up when transmitting not receiving the result is basicly explained below. A miss-matched antenna will work for reception but not for transmission. The best example of this is how often have you seen a coat hanger used for a car radio, I wouldn't want to try and transmit with one though.

The comment regarding checking your connects with a SWR meter are completely correct, it is quite possible to have a miss-match which would cause the problems you mention. If you have this problem I would cease transmitting NOW if there is such a problem the transmitted power is reflected back to the radio circuits and will burn out the radio (not a cheap repair if possible at all).

I would suggest that you get an expert to check the installation, It's a 5 min job to check with the right kit (possibly a lot more to repair).

I hope you find the solution, but remember there are thousands of Amateur radio operators who spend hours trying to eliminate these type of problems as a hobby

One last word of warning is do not touch the antenna when it is transmitting RF burns badly.

 

[john_travers]

Is it possible that particular types of antenna don’t work very well with particular types of VHF set?
In my case a Swiftech 168 and a masthead whip. An often seen combination.
Could insufficient power / voltage to the radio cause the problems?
Just another question re RF can burn. Presumably no danger from handhelds, is it just 25 watts that can cause damage?

 

[Boatman]

John

The general answer is no you shouldn't have a problem with the antenna as long as it was designed as a marine VHF antenna. As to power issues yes they can cause problems if there is a short in the system or it is being overloaded. RF burn is only a problem with bare wire antennas and lots of power(watts) so no problem with handhelds.

 

[Guest]

First check you are on 25 watts.vhf is line of sight so the higher each aeriel the longer the distance.Usualy the Coast Guard have an aeriel high up so they can transmit a long distance and when you transmit to them they should hear you.To check your power output you need a dummy load which is a 50 ohm 25watt resistor this is plugged into your aeriel output on the set in place of the coax. and the meter indicates the output.If you get 1watt &25 watts in each switch position the fault lies in the aeriel system,which is usually the case,most problems are with the aeriel.

double trouble